For example somebody who views Mt Rushmore for the first time even if he/she doesn't know the designer he/she will immediately correctly assume it was designed
Only because Mt Rushmore is carvings of human heads. In other words, it is something we are all familiar with. This is a weak argument.
I guess I'm just not getting the whole ID thing. What is the criteria for 'looking designed'? It seems to me it's just based on human perception and that's why you keep bringing up Mt Rushmore.